Threaded joints for steel pipes, such as oil country tubular goods and riser pipes, which are widely used in installations of the oil-producing industry are constituted by a pin which is a male threaded element provided on the end portion of a first tubular member and a box which is a female threaded element provided on the end portion of a second tubular member. Connection is carried out by threaded engagement of the male thread and the female thread, which are both tapered threads.
The first tubular member is typically a pipe such as an oil country tubular good, and the second tubular member is typically a separate member in the form of a coupling. (This type of threaded joint for steel pipes is referred to as a coupling type.) In this type, a pin is formed on both ends of the pipe, and a box is formed on both sides of the coupling. There are also integral threaded joints for steel pipes in which a pin is formed on the outer surface of one end of a pipe and a box is formed on the inner surface of the other end of the pipe. In this type, the first tubular member is a first pipe and the second tubular member is a second pipe.
In the past, oil country tubular goods were connected primarily using standard threaded joints prescribed by API (American Petroleum Institute) standards. However, in recent years, as the environments for excavation and production of crude oil and natural gas are becoming more severe, high-performance special threaded joints referred to as premium joints are being increasingly used.
In a premium joint, the pin and the box each have, in addition to a tapered thread which enables connection, a sealing surface provided on the peripheral surface in the vicinity of the threaded portion and a shoulder surface which functions as a stopper which undergoes abutment during make-up of the joint. Radial interference is provided between the sealing surfaces of the pin and the box. When a threaded joint is tightened until the shoulder surfaces of the pin and the box abut each other, the sealing surfaces of these two members intimately contact each other around the entire periphery of the joint and form a seal by metal-to-metal contact. In addition to acting as abutting stoppers at the time of make-up, the shoulder surfaces perform the function of bearing a compressive load during use of the joint.
FIG. 1 is a schematic explanatory view of a typical premium threaded joint for steel pipes of the coupling type. FIG. 1(A) is an overall view, and FIG. 1(B) is an enlarged view of a portion thereof. As shown in FIG. 1(B), this threaded joint for steel pipes has a pin 1 which is a male threaded element provided on the end of a pipe and a box 2 which is a corresponding female threaded element provided on both sides of a coupling. The pin 1 has on its outer surface a tapered male thread 11 and an unthreaded cylindrical abutting portion 12 called a lip (referred to below as a lip portion) provided on the end of the pin and adjoining the male thread 11. The lip portion 12 has a sealing surface 13 on its outer peripheral surface and a shoulder surface 14 on its end surface. The sealing surface 13 is a tapered surface (frustoconical surface) which gradually decreases in diameter towards the end of the pin.
The opposing box 2 has on its inner surface tapered female thread 21, a sealing surface 23, and a shoulder surface 24 which can threadingly engage with or contact or abut with the tapered male thread 11, the sealing surface 13, and the shoulder surface 14, respectively, of the pin 1.
As shown in the drawings, the lip portion, which has a shoulder surface on its end, is mostly provided on the end portion of the pin. In addition to serving as a stopper (torque shoulder) during tightening, the shoulder surface bears compressive loads acting on the joint.
When vertical wells were predominant, a threaded joint for steel pipes had sufficient performance if it could withstand the tensile loads due to the weight of the pipes connected thereto and could prevent leaks of high pressure fluid flowing along its interior. However, in recent years, wells are becoming deeper and the number of sloping wells or horizontal wells which bend or curve under the ground increases. In addition, oil wells in severe environments such as in the sea or polar regions are being increasingly developed. Accordingly, there is a strong demand for threaded joints for steel pipes to have increased resistance to compression and increased sealing performance particularly in the presence of internal and external pressure.
When external pressure acts on the above-described conventional premium joint, the applied external pressure is transmitted along gaps between the threads and permeates to the location indicated by 31 in FIG. 1(B) just before the sealing surfaces. Since the lip portion 12 has a considerably smaller wall thickness than the pipe body, it sometimes undergoes deformation toward reduction in diameter by the penetrated external pressure. As a result, if the external pressure increases, a situation may develop in which a gap forms between the sealing surfaces, leading to leakage, namely, penetration of exterior fluid to the interior of the pipe body.
When a compressive load acts on a premium joint, such as when an oil country tubular good is installed underground in a horizontal well or a sloping well, with most joints, the gap between the male thread and the female thread is made relatively large in order to facilitate stabbing, as a result of which the ability of the threads to bear a compressive load is low. Therefore, a compressive load is primarily borne by the shoulders. However, the wall thickness of the shoulder surface at the end of the lip portion (the pressure-receiving area for compressive loads) is normally much smaller than that of the pipe body, so when a compressive load corresponding to 40-60% of the yield strength of the pipe body is applied, the lip portion of most premium joints undergoes large plastic deformation, and the sealing ability of the adjoining sealing surface markedly decreases.
In order to increase the sealing ability of a joint against external pressure (external pressure sealing ability), the stiffness of the lip portion of the end of the pin can be increased so as to increase its resistance to deformation toward a reduction in diameter. With this object, a method referred to as swaging in which the wall thickness of the lip portion is increased by axial constriction at the end of the pipe is often used.
However, if the amount of swaging is too great, in the case of connecting casings, a pipe inserted into the interior of the connected casings sometimes catches on the swaged portion, and in the case of connecting tubings, the swaged portion causes turbulence in the fluid such as crude oil which is flowing inside the connected tubing, and this turbulence sometimes causes erosion. Therefore, the amount by which the wall thickness of the pin lip can be increased by swaging is limited.
Other conventional techniques for increasing the stiffness of the end of a pin and increasing sealing performance are described in below-mentioned Patent Documents 1 and 2. These patent documents disclose technology in which sealing performance is increased by installing a cylindrical portion which does not contact the box at the end of the sealing surface of a pin so as to increase the stiffness of the periphery of the sealing surface of the pin against diameter-reducing deformation and producing uniform contact between the sealing surfaces of the joint.
Even if the wall thickness of the lip portion of a threaded joint for pipes is increased by swaging, it is necessary for the threaded joint to provide tapered threads, a sealing surface, and a shoulder surface within a limited wall thickness. However, according to the technology disclosed in the above-described patent documents, the end of the pin does not abut the box, so the shoulder surface must be provided in a location other than the lip, and the lip necessarily becomes thin. Accordingly, there is a limit to the extent to which the stiffness of the lip can be increased so as to resist diameter reducing deformation of the lip caused by external pressure, and a high degree of sealing ability against external pressure cannot be achieved. In addition, because the shoulder surface of the pin cannot be given a sufficient wall thickness, high resistance to compression cannot be obtained, and the sealing performance under a combined load of compression and external pressure is low.
As shown in FIG. 2, below-mentioned Patent Document 3 proposes a threaded joint for steel pipes in which, a nose portion 15 which is an extended lip portion is provided between the sealing surface 13 of a pin 1 and the shoulder surface 14 at the end of the pin. The nose portion 15 of the pin 1 is a noncontacting region which does not contact the opposing portion of the box 2. At the same time, the sealing surfaces 13 and 23 and the shoulder surfaces 14 and 24 of the pin and the box contact each other. By extending the lip portion of the pin 1 so as to provide a nose portion 15 having an outer surface with a cylindrical shape (having a constant outer diameter) which does not contact the box in a position closer to the end of the pin than the sealing surface 13 of the pin, the wall thickness of the lip portion having a limited pipe wall thickness and accordingly the wall thickness of the shoulder surface and the sealing surface of the pin can be increased, whereby the resistance to compression and the sealing properties against external pressure of a threaded joint for pipes can be markedly increased.
In the threaded joint shown in FIG. 2, in order to allow the male thread 11 to approach as close as possible to the sealing surface 13, a circumferential groove 32 is provided in the box such that the male thread in a portion closest to the end of the pin does not threadingly engage with the female thread in a position in the vicinity of the sealing surface of the box.
A threaded joint for steel pipes like that proposed in Patent Document 3 which has a noncontacting region on the side of the sealing surface of a pin closest to the end of the pin exhibits adequate sealing performance and resistance to compression even when it receives a tensile or compressive load applied thereon in a state in which internal or external pressure is applied to the threaded joint. However, it has a relatively long nose portion on the side of the sealing surface closest to the end of the pin, with the sealing surface having a tapered shape with an outer diameter which gradually decreases towards the end of the pin. Therefore, if the tip of the pin impinges on the box at the time of joint make-up or if it previously impacts some other object, the tip of the pin may deform so as to swell outward. As a result, there is the possibility of it being impossible to further tighten the pin, or even if stabbing and tightening are possible, the sealing properties of the sealing surface may be impaired by the swelling of the tip of the pin such that the gastightness (the sealing performance) of the threaded joint for steel pipes is worsened.
In particular, as shown by 16 and 26 in FIG. 2, when a chamfer is provided on the inner peripheral surface in the vicinity of the shoulder surfaces of the pin and the box with the object of preventing turbulent flow of the fluid flowing in the interior of the joint, the wall thickness of the end of the nose portion becomes thin, and deformation of the end of the pin due to impact with the box occurs more easily.
As the nose portion of the end of the pin is relatively long, there is the danger of the sealing surface of the box being damaged due to deviation of the stabbing angle at the time of stabbing of the pin. In addition, if the outer peripheral surface of the end of the pin is a substantially cylindrical surface, a guide in the direction of stabbing of a pin for correcting deviation of the pin axis with respect to the box axis is not obtained.    Patent Document 1: U.S. Pat. No. 4,624,488    Patent Document 2: U.S. Pat. No. 4,795,200    Patent Document 3: WO 2004/109173